1. Field of the Invention
The invention relates to a rotary cutting tool with chip breakers and, more particularly, to a rotary cutting tool or the like in which the metal removing teeth have been modified so as to enable the tool to be operated at high speeds to form a smoother cut with less pressure and heat generation than is possible with conventional cutters and also to reduce power requirements associated with such a rotary cutting tool.
2. Description of the Prior Art
For illustrative purposes, the specification will described the invention as it pertains to a conventional end will, i.e., a milling cutter of cylindrical configuration having a shank portion and a cutting portion, the cutting portion containing a plurality of helically disposed cutting blades extending from the shank portion of the milling cutter to the free end thereof. In such a milling cutter, the cutting edges of the blades lie on a substantially constant radius with respect to the longitudinal axis of the tool. However, the invention may also have application to tapered end mills wherein the cutting portion is generally frustoconical in form, and the cutting edge of each tooth has a constantly decreasing radius with respect to the longitudinal axis of the tool as the edge extends from the shank end of the cutting portion to the free end. Also in such a tapered end mill, the cutting edges of the teeth are at the same radius from the longitudinal axis of the tool in any plane through the cutting portion and perpendicular to the longitudinal axis of the tool. The invention also has application to the so called "straight fluted" end mill wherein the teeth extend parallel to the longitudinal axis of the tool, as opposed to helically with respect thereto and, of course, the invention may also be utilized with other forms of rotary cutting tools which are not properly categorized as end mills or milling cutters.
There are several inherent problems in the use of the conventional milling cutter as described above. Generally, these problems manifest themselves in excessive wear and relatively poor cutting actions, or both, due to the fact that the entire length of the cutting edge may be applied to the workpiece at the same time, and due to the fact that continuous chips are produced which are not satisfactorily removed from the work area. There have been many attempts to improve the cutting action and decrease the wear in such tools, and these attempts usually involve the use of so called "chip breakers" in the form of relatively deep notches cut transversely into the cutting blade at spaced intervals, or some similar form of providing an interrupted cutting edge along each blade. One such form as hereinabove referred to is a "chip breaker" described in Erhardt, U.S. Pat. No. 2,855,657, issued Oct. 14, 1958, which discloses that the cutting edge of each blade is provided at spaced intervals along each helical blade with notches of substantial depth which are ground therein for the purpose of interrupting the chips produced by the plurality of helically disposed parallel cutting teeth. It is further noted that the notches in successive teeth circumferentially of the tool are slightly axially offset, this effect preferably being obtained by grinding such notches in a low pitch helical path circumferentially of the tool. Erhardt further discloses a shallow bevel merging with each such notch and formed on one side of the notch of one tooth and on an opposite side of such notch of the next adjacent tooth so that they alternate first to one side and then to the other. The purpose of such arrangement is to balance out the endwise thrust on the tool and to maintain the torque more nearly centered. This attempted solution to the aforementioned problems has met with some success in improvement of tool life and in facilitating chip removal from the work area, although such success is due primarily to the fact that smaller, discontinuous chips are produced which may be more readily removed from the work area. In other words, the solutions have been directed primarily to a form of the chip produced, rather than removal of the chips from the work area. Also, in the construction as suggested by Erhardt wherein the helical cutting edges are interrupted axially of the body by one or more helical grooves which spiral about the body at either the same or different pitches as the flutes, but in opposite directions, that is, the flutes spiral in the right-hand direction, whereas the grooves spiral in a left-hand direction, the grooves then are disposed in a direction which impedes chip removal. That is, a chip at the leading edge of a cutting tool defined by a groove normally tends to move towards the shank end of the cutter along the helix of the flute, but if the chip enters the groove, it is urged towards the cutting end of the cutter. As a result, the pressure relief advantages caused by grooves are dissipated to some extent, the tool operates at a higher temperature, and the smoothness of the cut suffers.
Several attempts have been made in the prior art to solve this problem. For example, Cave et al, U.S. Pat. No. 3,548,476, discloses a cutter having a plurality of helical flutes of uniform length and depth which form a plurality of helical cutting edges circumferentially spaced from one another by the flutes and which spiral about the axis of the body in the same direction at the same pitch as the flutes. Each of the cutting edges extend radially of the body and merge smoothly with its associated cutting edge which spirals in a right-hand direction about the axis of the body at a predetermined angle. These cutting edges are interrupted longitudinally at spaced intervals by a plurality of notches. The notches are formed by a groove which spirals about the axis of the body in the same direction (right-hand helix for both flutes and notch groove) of the flutes but at a steeper pitch. The helix on which the notches are formed and the width of the notches in the teeth should be so selected that the circumferentially adjacent teeth on successive cutting edges are offset axially from one another by an amount such that each circumferentially successive tooth has a portion which follows a portion of a groove in a preceding cutting edge. The amount of offset between successive teeth should be such that, for each complete revolution of the body, the tooth trailing any given notch will more than offset the length of the notch. A cutting tool construction in accordance with the disclosure of Cave defines advantages in that the formation of discontinuous chips facilitates chip removal and the spiraling of the notches in the same direction as the flutes further facilitates chip removal. Moreover the discontinuous cutting teeth result in less drag or resistance to rotation of the tool when it is in operative engagement with a workpiece, thereby reducing deflection of the tool and permitting cutting of the workpiece to closer tolerances than would otherwise be possible, enabling the tool to operate at a relatively low temperature and thereby increasing tool life. The specific problems inherent in Cave, however, that is, the notches which form the groove which spirals about the axis of the body in the same direction as the flutes generate a strong longitudinal force during a cut and, eventually, as the tool dulls this force will become sufficient to pull the cutting tool from its workholding device.
Minicozzi, U.S. Pat. No. 4,212,568, is also directed to the problem of facilitating chip removal from the work area while improving the tool life. Minicozzi discloses a conventionally constructed cutting tool as set forth above wherein during the spiral forming of the flutes in the cutting portion of the cutting tool a template is used which, as the fluting mill is moved along a spiral path from one end of the cutting portion to the other, causes the flute mill to move alternatively towards and away from the longitudinal axis of the tool to be formed along an undulating or sinusoidal path so that each tooth will have a cutting face and a trailing face with surfaces which undulate generally sinusoidally from one end of the cutting portion to the other. Minicozzi further discloses that the leading edge and trailing edge of each blade is interrupted by a plurality of relatively shallow transverse depressions of relatively large radius arcuate cross-section resulting in cutting edges with a variable rake angle which tends to reduce tool wear. Further, the sinusoidally undulating surface of the cutting face of each tooth gives rise to a rake angle at each cutting edge which varies continuously along the length of the cutting edge, facilitating the formation of relatively small chips. When these chips move through the space where the cutting and trailing face surfaces are convex they tend to squeeze at these areas and tend to spring away from the workpiece and the cutting tool when free to do so, in this way facilitating chip removal. The cutting tool disclosed in Minicozzi is time consuming to manufacture and, therefore, expensive. Additionally, its chip removing feature provides little improvement to the feed and speed characteristics of the cut.
Shanley, Jr., U.S. Pat. No. 4,285,618, recognizes the need for rapid cutting and the requirement of smooth dimension finishing qualities simultaneously. Accordingly, Shanley, Jr. discloses a conventional cutting tool of a hard metal body whose cutting portion surface is formed into a plurality of blades separated by flutes, each blade having a leading side, a cutting edge on the leading side, a land, and a trailing side of face; at least two of the blades having at least one smooth segment, wherein the land and cutting edge are even and unbroken, and at least one serrated segment formed with crests that are flat, round, or sinusoidal, wherein the land consists essentially of a row of cutting teeth, adjacent teeth in the lands being separated from each other by a transverse groove in the blade. The smooth end serrated segments are located in staggered positions from blade to blade, so that in the course of one complete revolution of the cutter, each point along a surface being formed by the cutter will be contacted by at least one smooth segment and at least one serrated segment. The serrated segments are arranged in such a pattern from blade to blade that continuous, imaginary line passing across each blade at precisely the midpoint of each serrated segment would define a helix of uniform angle around the cutting section measured from a line which is parallel to the shank section. Shanley, Jr. further disclosed that the helix may be right-handed or left-handed such that the helix of the serrated segments is the same as the helix of the blades on the cutting portion of the tool. Generally, the cutter will be capable of faster metal removal if the lay of the helical serration pattern is opposite that of the helical pattern of the blades. Shanley Jr. discloses that the teeth in the serrated segments may be formed with flat, rounded or sinusoidal crests. This type of tooth formation, however, will result in drag and galling of the leading edge of the cutting tooth since the leading edge of the cutting tool has a negative rake angle.
Kishimoto, U.S. Pat. No. 4,497,600, discloses an end mill wherein the shape and the arrangement of the notch required for the blade can be freely selected without using thread cutting. Kishimoto disclosed a cutting tool wherein along the whole circumferential surface of each blade, notches are machined at prescribed intervals, extending transverse to the length of each blade. Each notch is shifted slightly along the blade toward the tool end or the shank end relative to a corresponding notch on the preceding blade. When X is taken as the width of the blade surface between notches, Y as the width of the notch, and Z as the amount of shift of a notch relative to the corresponding notch in the adjacent preceding blade, the shift (Z) of the notch is at least equal to (X+Y)/(N) (where N is the number of blades). In the case of shifting the notch toward the tool end, the inclination of the notch on its own axis is toward the shank end and when shifting the notch toward the shank end, the inclination of the notch on its own axis is toward the tool end. Since the notches are formed at the prescribed intervals and with a shift of a prescribed amount, rather than being formed along a helical path as in a screw thread, the side clearance of the notch is not limited by the number of blades. Since the direction of the side clearance is varied by the direction of the shift of the notches relative to the preceding and succeeding blades, ". . . the cutting by the main blade is perfectly formed with an important effect of elevating the cutting performance. "The angle of the notch and the shape of the notch can be freely selected depending on the material to be cut. In the conventional tool in which the notch is formed along a helix, such as a thread, these values are fixed. The disclosure of Kishimoto results in a notch in a blade which will have a negative rake angle on the leading or trailing edge of the tooth form. The negative rake angle causes drag and galling of the metal at the cutting edge of the tooth as well as heat build up and higher wear or shorter life of the cutting tool.
What is needed, therefore, is a cutting tool which overcomes the disadvantages of the prior art and which recognizes the advantages of applying chip breaking features in a helix which is opposite hand of the flute helix and hand of cut and which utilizes a chip breaking notch relief angle to generate a positive relief angle on the cutting tooth edges at the point of cutting to maximize the metal removal with a resultant desired close dimensional finishing quality while also realizing an increase in power efficiency to drive the tool through the work.